The Finder and its related Open File dialog help you preview images and other documents so that you can decide which to open, and so on. However, their controls aren’t always obvious. This short article explains how to access them.

In Finder windows, controls over the size of previews and thumbnail icons are more obvious.

In Column views, the Quick Look preview of the selected document is fitted to the free space at the right of that Finder window. To enlarge it, simply drag the window out to a larger size. This is limited by the number of columns you have on display: if you are working quite deep in the folder hierarchy, you may find it helpful to place an alias at a higher level, or to add that folder to your Favourites listed in the sidebar.

In Icon views, the Finder has a slider available in the Status Bar, at the bottom right of the window. If you don’t already have the Status Bar enabled, you can toggle it by pressing Command-/ Then simply drag that slider until the icons are sufficiently large.

You can always select an image (or other file) and press the Spacebar to see it in Quick Look’s full preview, when you need to see even more detail.

In Open File dialogs, the controls are more limited, and one is hidden completely.

In Column view, previews are again fitted to the free space at the right of the dialog, so dragging the dialog window out to a larger size will provide a larger preview with better detail. You can still press the Spacebar to see a larger Quick Look preview too.

In Icon view, there is no slider control, nor any option for one. Pressing the Spacebar still opens a larger Quick Look preview, but when you’re searching for the right document that is not always ideal.

You can change the size of the thumbnail icons used in the Open File dialog (Icon view), but unless you have a third-party utility which offers this as one of its settings, you will need to do this in Terminal’s command line.

The command that you should use is of the form
/usr/libexec/PlistBuddy -c 'Set FK_StandardViewSettings:IconViewSettings:iconSize 128' ~/Library/Preferences/com.apple.finder.plist

That uses the rather old and decrepit built-in tool PlistBuddy to change the icon size in Finder’s preferences from its default of 64 x 64 pixels, in this example to 128 x 128 pixels. Choose a multiple of 64 for the best results, and beware that too large a size will also be unusable.

When you have changed that setting, you will need to force Finder to quit and restart (using the Force Quit dialog), log out and back in again, or possibly even restart, for the new setting to take effect; user experience varies. As this is an undocumented feature, it may stop working at any time, or produce unexpected results. With a bit of luck, though, it should do the job.

Thanks to Gene for drawing attention to this, and for the solution to the puzzle of how to enlarge icons in the Open File dialog.